In an optical transmission system, an apparatus that monitors an optical signal-to-noise ratio (OSNR) is now in practical use to detect the state of an optical transmission apparatus, an optical transmitter and receiver, an optical transmission lines, and the like or to detect a failure therein. Since a structure in which one or both of the wavelength and path of an optical signal are dynamically changed will be used in a next-generation optical network, demands for monitoring optical signal quality can be thought to further increase.
When an OSNR monitor is mounted, it is desirable for a main signal (for example, an optical signal that transmits data) not to be affected. Specified monitoring precision is desirable in a case in which polarization multiplexing is performed or dispersion (such as wavelength dispersion or polarization mode dispersion) is present, or even in a case in which spectral narrowing occurs. In addition, a simple and inexpensive structure is desirable.
In an example of an OSNR monitor implemented with a simple structure, the structure is proposed as described below. An optical splitter leads an optical signal to a first path and a second path. An optical power measuring unit measures the strength of the optical signal on the first path. A noise measuring unit processes an alternate-current component of the optical signal that has selectively passed on the second path, and measures the strength of the noise of the processed alternate-current component. An OSNR calculating unit calculates the OSNR of the optical signal by comparing the measured signal strength with the measured noise strength. (Related technologies are described in, for example, U.S. Pat. No. 6,433,864 and Japanese Laid-Open Patent Publication Nos. 2004-287307 and 2009-244163.)
In the structures in the above related technologies (for example, the structure described in U.S. Pat. No. 6,433,864, it is desirable to obtain, in advance, a coefficient that is used to calculate an OSNR according to the measured signal strength and measured noise strength (the coefficient will be referred to below as the calibration coefficient). This calibration coefficient can be calculated by, for example, adding known amplified spontaneous emission (ASE) noise to an optical signal and measuring the OSNR of the optical signal with an optical spectrum analyzer. In an actually created optical transmission system, however, a special facility is used to add ASE noise to an optical signal and measure an OSNR with an optical spectrum analyzer. Another problem is that extra work and a cost are involved.
An object in an aspect of the present disclosure is to provide an apparatus and a method for creating a calibration coefficient used to monitor an optical signal-to-noise ratio with a simple structure.